1. Field of the Invention
The present invention relates to a color misregistration easing system for easing color misregistration derived from field sequential imaging.
2. Description of the Related Art
An endoscope whose elongated insertion tube is inserted into a living body to observe a lesion or other subject has been adopted widely in recent years.
An electronic endoscope system in which a CCD or other solid state imaging device is used as an imaging means has also come into practical use recently.
The electronic endoscope system mentioned above includes a field sequential type electronic endoscope in which a subject is illuminated successively with illumination light of red, green, and blue rays having different wavelengths, and images produced under the rays of different wavelengths or component images are synthesized to produce a color image.
In such a field sequential type imaging system, unlike a simultaneous type imaging system in which white light is used as illumination light and a mosaic color filter is installed on the front of an imaging plane of a solid state imaging device, the solid state imaging device uses a larger number of imaging cells to receive light of one wavelength. Therefore, the imaging plane system permits higher resolution.
However, in the field sequential system, component color images that are produced at different times are synthesized to form a signal color image. Therefore, when a moving subject is imaged or an imaging means and a subject are moving relatively, a synthetic color image appears with colors different from the original colors of the subject.
In an electronic endoscope system, a mucosa may adhere to an objective at the tip of an endoscope. This disables accurate observation. In this case, water supply is executed; water will be jetted toward the objective from the distal end of the endoscope to clean the lens. During the water supply, water runs fiercely in front to the objective. Therefore, when the water is illuminated with the field sequential illumination light, it is seen with drastically-changed colors or is rainbow-colored. In some endoscopes, the water supply is carried out to wash or cool a region to be examined. FIG. 23 shows the endoscopy during water supply.
The principles of causing a color reproduction error will be described in conjunction with FIG. 23. When water is jetted from the distal end of an endoscope, the water runs quickly in front of an objective. When red (R) light is, for example, radiated, the water is visualized as red water (object) just coming out of the distal end of the endoscope. When green (G) light is radiated, a waterdrop moves on the lens surface and appears as a green object. When blue (B) light is radiated, the water is visualized as blue water that has moved farther. These color images are synthesized and displayed on a monitor.
Therefore, the water image is displayed in primary colors of red, green, blue, yellow, cyan, and magenta with high saturation. In short, the water image is rainbow-colored. Consequently, a doctor undertaking endoscopic examination suffers fatigued eyes. Hereafter, a color reproduction error resulting from a difference in time of sampling color information is referred to as color misregistration. The event causing the color misregistration includes the aforesaid movements as well as zooming.
In Japanese Patent Laid-Open No. 2-70644, the present applicant has proposed that color component signals are checked to identify color misregistration, and that when color misregistration is identified, a correction color signal produced with an actual luminance is supplied to ease the color misregistration.
In Japanese Patent Laid-Open No. 2-27843 (U.S. Pat. No. 5,032,913), the applicant has proposed incorporation of a water supply detecting means that receives an image signal from an electronic endoscope, and identifies color misregistration derived from the water supply. Thereby, only when it is determined that the water supply is under way, a correction color is displayed. Thus, only intense color misregistration is eased.
Alternatively, Japanese Patent Laid-Open 3-24513 has disclosed a color misregistration detector in which color difference signals of a current image are generated to calculate a hue, then the hue component is held.
An image produced during water supply or other image causing conspicuous color misregistration has a high saturation. Even in a general image, a bleeding region appears with a relatively high saturation.
However, in the aforesaid color misregistration detecting method, since only a difference of a hue component is detected to identify color misregistration, even negligible color misregistration is subjected to color misregistration easing. Therefore, color misregistration is detected at a level inconsistent with a level at which a sensory test has revealed that an observer discerns color misregistration. That is to say, even negligible color misregistration is identified as color misregistration. Even for color misregistration with which an observer is not annoyed, color misregistration easing is carried out.
In Japanese Patent Laid-Open No. 2-27842 (U.S. Pat. No. 5,032,913), data is kept updated even when intense color misregistration has occurred. When intense color misregistration occurs, average color differences of a screen (one field or one frame), which are used to produce the correction color, varies incessantly. Therefore, a stable correction color is unavailable.
The system disclosed in the Japanese Patent Laid-Open No. 2-27842 (U.S. Pat. No. 5,032,913) always applies an average color irrelative to a degree of color misregistration.
A system for easing color misregistration should, preferably, replace only an area of color misregistration in an image with a correction color. To detect the area of color misregistration and correct only the area, memory circuits and other digital circuits must be incorporated. This results in a large-sized and high-cost circuitry.